A sensor for detecting the presence of a passenger in a vehicle sitting in a seat having a back rest is known from European Patent 484 293 B1, the sensor having an infrared-sensitive element and a lens system and being situated in the roof liner above the seat. The heat radiation of a passenger is detected by the infrared-sensitive element, allowing a conclusion to be drawn regarding the presence of a passenger.
A device for detecting the presence or the body position of a passenger in which an infrared transmitter and an infrared receiver are accommodated in a housing situated in the roof liner is known from U.S. Pat. No. 6,113,137. The infrared light, emitted by the infrared transmitter and reflected by the seat or by any passengers present, is picked up by the infrared receiver in the common housing and is analyzed. Within the scope of analyzing the intensity of the light reflected or its variation, the size class of the passenger (small, big) and the class of the sitting position (very dangerous position, possibly dangerous position, correct position) are identified or the presence of a passenger is detected.
Furthermore, a method of determining the body position in which the length by which a seat belt is rolled off vis-à-vis its rest position is determined is known from German Patent Application 197 49 855 A1. The body position of the passenger is identified based on this length.
These methods described have the common feature that they either operate very inaccurately, or are very complex in their analysis or in the implementation of the object